The use of three-dimensional environments for cell culture provides a more physiological relevant system for in vitro modeling of cell behavior and for the creation of constructs for subsequent implantation. In the body, tissues are composed of multiple cell types and cells are organized in specific spatial arrangements providing orientation of cells into geometries specific to organ functions. The study of cell function in vitro, originally utilizing cells grown on tissue culture surfaces (e.g. glass and plastic) has now transitioned to three-dimensional cultures of cells that are often embedded in collagen gels. Coordinately, investigators have evaluated the ability of two- and three-dimensional cell cultures to undergo the spontaneous formation of spheroids during culture. Epithelial and endothelial organoid cultures have been established in this way [1-5]. In those procedures, embryonic stem cells were cultured as hanging drops and allowed to form embryoid bodies (EBs) [6, 7]. Spheroid culture strategies have since progressed to include endothelium, representing cells of the vasculature, a common cellular component of all complex tissues [8-10]. And recently, complex three-dimensional tissue constructs containing parenchymal cells and vascular cells have been implanted in experimental models [11, 12]. Each of those studies show that functional tissue organoids can be constructed in vitro, implanted in tissue with evidence of vascular integration between implanted and recipient circulations and with evidence that the organoids can provide restoration of tissue function.
The formation of three-dimensional cell and tissue constructs, however, has yet to be fully evaluated and realized using bioprinting technologies [13-16]. Bioprinting, the biologic equivalent of Computer Assisted Design (CAD) and subsequent Computer Assisted Manufacturing (CAM) technologies, includes several different fabrication systems including direct-write bioprinting and ink jet bioprinting [13, 17, 18]. These systems provide CAD-CAM based methods for the controlled deposition of biological materials toward the fabrication of complex biological structures. As such, any improvements to the use of bioprinting for the production of such complex biological structures would be both highly desirable and beneficial.